Preparation of a stock solution of free chlorine:
First prepare a 1:10 dilution using a sodium hypochlorite solution containing approximately 6 - 14 % of active chlorine. For this pipette 10 ml of sodium hypochlorite solution into a calibrated or conformity-checked 100-ml volumetric flask and then make up to the mark with water for analysis.

Precise assay of the stock solution for free chlorine:
Pipette 10.0 ml of the stock solution into a 250-ml ground-glass-stoppered conical flask containing 60 ml of water for analysis. Subsequently add to this solution 5 ml of hydrochloric acid 25 %. And 3 g of potassium iodide. Close the conical flask with the ground-glass stopper, mix thoroughly, and leave to stand for 5 min. Titrate the eliminated iodine with sodium thiosulfate solution 0.1 mol/l until a weakly yellow color emerges. Add 2 ml of zinc iodide starch solution and titrate from blue to colorless.

Calculation:1 ml sodium thiosulfate solution = 3.55 mg free chlorine
Further investigational concentrations may be prepared from the stock solution prepared according to the procedure described above by diluting accordingly.

Stability:
When stored in a cool place (refrigerator), the stock solution remains stable for approximately one week. The diluted investigational solutions are stable for approximately 2 hours.